An intensive care unit (ICU), also known as an intensive therapy unit or intensive treatment unit (ITU) or critical care unit (CCU), is a special department of a hospital or health care facility that provides intensive treatment medicine.

Intensive care units cater to patients with severe and life-threatening illnesses and injuries, which require constant, close monitoring and support from specialist equipment and medications in order to ensure normal bodily functions. They are staffed by highly trained doctors and nurses who specialise in caring for critically ill patients. ICUs are also distinguished from normal hospital wards by a higher staff-to-patient ratio and access to advanced medical resources and equipment that is not routinely available elsewhere. Common conditions that are treated within ICUs include acute (or adult) respiratory distress syndrome (ARDS), trauma, multiple organ failure and sepsis.

Patients may be transferred directly to an intensive care unit from an emergency department if required, or from a ward if they rapidly deteriorate, or immediately after surgery if the surgery is very invasive and the patient is at high risk of complications.

Specialities
Hospitals may have ICUs that cater to a specific medical speciality or patient, such as those listed below:

Neonatal intensive care unit (NICU). This specialty unit cares for neonatal patients who have not left the hospital after birth. Common conditions cared for include prematurity and associated complications, congenital disorders such as Congenital diaphragmatic hernia, or complications resulting from the birthing process.
Pediatric intensive care unit (PICU). Pediatric patients are treated in this intensive care unit for life-threatening medical problems such as asthma, influenza, diabetic ketoacidosis, or traumatic brain injury. Surgical cases may also be transferred to the PICU postoperatively if the patient has a potential for rapid deterioration and requires more frequent monitoring, such as in spinal fusions or surgeries involving the airway such as removal of the tonsils or adenoids. Some facilities also have specialized pediatric cardiac intensive care units, where patients with congenital heart disease are cared for. These units also typically handle cardiac transplantation and postop care of cardiac catheterization patients if those services are offered at the hospital.

Psychiatric intensive care unit (PICU). Patients who may voluntarily harm themselves are delivered here so they can be monitored more vigorously. Patient rooms are locked, preventing escaping.

Coronary care unit (CCU): Also known as Cardiac Intensive Care Unit (CICU) or Cardiovascular Intensive Care Unit (CVICU), this ICU caters to patients specifically with congenital heart defects or life-threatening acute conditions such as cardiac arrest.

Neurological intensive care unit (NeuroICU). Patients here are treated for aneurysms, brain tumors, stroke, rattlesnake bites and post surgical patients who have undergone various neurological surgeries and require hourly neurological exams. Many nurses who work within these units have neurological intensive care certifications. Once the patients are more stable and off the ventilator, they are transferred to a neurological care unit.
Trauma intensive care unit (Trauma ICU). These are found only in hospitals certified in Trauma and have a dedicated Trauma Emergency Department equipped with a team of surgeons, nurses, respiratory therapists, and radiological staff.

Post-anesthesia care unit (PACU): Also known as the post-operative recovery unit, or recovery room, the PACU provides immediate post-op observation and stabilisation of patients following surgical operations and anesthesia. Patients are usually held in such facilities for a limited amount of time, and must meet a set physiological criteria before transfer back to a ward with a qualified nurse escort. Due to high patient flow in recovery units, and owing to the bed management cycle, if a patient breaches a time frame and is too unstable to be transferred back to a ward, they are normally transferred to a high dependency unit (HDU) or post-operative critical care unit (POCCU) for closer observation.

High dependency unit (HDU): In the United Kingdom and elsewhere (known as step down unit or progressive care unit in North America), most acute hospitals have a transitional high dependency unit (HDU) for patients who require close observation, treatment and nursing care that cannot be provided on a general ward, but whose care is not at a critical enough level to warrant an ICU bed. These units are also called step-down, progressive and intensive recovery units and are utilised until a patient's condition stabilizes enough to qualify them for discharge to a general ward.

Surgical Intensive Care Unit (SICU): A specialized service in larger hospitals that provides inpatient care for critically ill patients on surgical services. As opposed to other ICUs, the care is managed by surgeons trained in critical-care.